1. Field of the Invention
The present invention is directed to a method of measuring defects in a glass article, and in particular, measuring the topography of thin glass sheets.
2. Technical Background
Flat panel displays, such as liquid crystal displays, are fast overtaking traditional cathode ray tube (CRT) display technology in the commercial arena. The manufacture of LCD display devices relies on thin sheets of pristine-surfaced glass, between which a liquid crystal material is sandwiched. Tolerances for surface defects for these glass sheets is extraordinarily stringent, requiring the ability to measure defects on a nanometer scale. Exacerbating the problem is the fact that the glass sheets are exceptionally thin, typically less than about 0.7 mm, and can be quite large—several square meters or more in some instances. As such large, thin sheets are quite flexible, maintaining the sheet flat, let alone stable (remaining in a given shape over a period of time), can be challenging.
Much effort has gone into developing appropriate fixturing, and measurement techniques that can measure defects quickly, thus reducing dependence on stability-related concerns (movement of the sheet over time). The measurement devices utilized for making nanometer-scale measurements of substrates typically include the use of an interferometer. Interferometers, such as the well-known Michelson interferometer, use interference between beams of light to create an interference pattern indicative of the difference in optical path length between the beams. This difference in path length can be used as an indicator of the topography of a surface under measurement.
One drawback of conventional area-scan techniques for the characterization of nanometer-scale surface defects on a specular surface is the need to maintain the measurement surface perpendicular to the probe or measurement beam while keeping within the angular tolerance of the interferometer. For this reason, the sample under test may be mounted on a movable stage which may be adjusted prior to performing the measurement. While this approach is applicable in a laboratory environment, or where small sample sizes are being measured (e.g. semiconductor wafers), in a production environment for processing large sheets of very thin glass, moving the sheet becomes prohibitive: movement of the sheet can itself create distortion of the sheet surface. Moreover, alignment of the sheet may potentially require repeated movement of the sheet to investigate potential defects over the surface. Movement of a large sheet can require complex, bulky equipment, and increase measurement time. Similar concerns accompany movement of the interferometer.
What is needed is a method and/or apparatus suitable for a production environment that enables nanometer-scale measurements of the surface topography of thin glass sheets, which may not be flat, without needless movement of the sheet or bulky measurement equipment.